With the heightened environmental concern regarding the presence of contaminants in drinking water has come the need to analyze water for volatile organic compounds generally. The purge and trap technique is a general purpose method for the identification and simultaneous measurement of purgable, volatile organic compounds in water that have sufficiently high volatility and sufficiently low water solubility to be efficiently removed from water. Among the volatile organic compounds which can be determined by the purge and trap procedure are benzene, bromobenzene, carbon tetrachloride, chloroform, cumene, naphthalene, styrene, toluene, the xylenes, vinyl chloride, tetrachloroethylene, hexachlorobutadiene, methylene dichloride and fluorodichloromethane. An analogous technique also is used for the analysis of volatile organic compounds in air.
In a typical purge and trap procedure, exemplified by EPA Method 524.2, volatile organic compounds and surrogates with low water solubility are purged (extracted) from the sample by bubbling an inert gas through the aqueous sample. Purged sample components are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with helium to desorb the trapped sample components into a capillary gas chromatography (GC) column interfaced to a mass spectrometer (MS). The column is temperature programmed to separate the analytes which are then detected with the MS. Compounds eluting from the GC column are identified by comparing their measured mass spectra and retention times to reference spectra and retention times in a database. Reference spectra and retention times for analytes are obtained by the measurement of calibration standards under the same conditions used for samples. A concentration of each identified component is measured by relating the MS response of the quantitation ion produced by that compound to the MS response of the quantitation ion produced by a compound that is used as an internal standard. Surrogate analytes, whose concentrations are known in every sample, are measured with the same internal standard calibration procedure.
The foregoing description was that for analysis of volatile organic materials in aqueous systems where the purge and trap technique is appropriate. However, it should be dear that an analogous procedure may be utilized for the analysis of volatile organic materials in, e.g., air analysis. The exposition within will be directed with particularity to analysis of volatile organics in aqueous media using the purge and trap procedure, but this is done solely for clarity and ease of exposition. It needs to be clearly understood that the subject matter is not restricted to such analyses, and is capable of significant expansion.
This application focuses on the sorbent tubes used in purge and trap analysis. In particular, our goal is the development of an improved sample concentration sorbent tube, superior to those presently available, to enhance the purge and trap procedure itself, both as to its methodology and its results.
The jet separator specified in, for example, EPA Method 524.2 for analysis using a GC/MS system can cause losses of 50% or more for small analytes, a condition alleviated somewhat by interfacing the column directly to a MS ion source. Elimination of the jet separator requires low column flow rates, which are not compatible with flow rates in purge and trap systems. Another option for improving sensitivity is the use of larger samples. Since both these options have significant disadvantages, we turned our attention to finding a sorbent tube considerably more efficient than those currently used.
Present adsorbents in the sorbent tubes used for purge and trap methodology appear to be one or more of various charcoals or porous carbons, organic polymers such as that of 2,6-diphenylene oxide (e.g., Tenax.RTM.), and silica gels. Each individually and even in combination suffer from distinct limitations and disadvantages. One disadvantage is that of limited capacity, so that "saturation" of the adsorbent is all too readily attained, leading to error in analytic results. Each also suffers from a lack of thermal stability, with temperatures of 200.degree. C. or so likely to lead to irreversible impairment as an adsorbent. Each additionally suffers from hysteresis or "memory" effects, i.e., complete desorption of some components may be difficult with additional desorption occurring during subsequent analyses using the same sorbent tube in a different purge cycle. This is frequently referred to as "carryover."
Perhaps the most severe limitation of present materials commonly used as adsorbents in purge and trap methods is their very limited linearity. That is, the adsorbents typically utilized by those in the art discriminate among the various classes of organic materials which may be present, and also may discriminate among the organic materials within a class. Thus, a substantial proportion of analysis time must be spent in calibrating sorbent tubes for their nonlinearity. What is instead desired are adsorbents which are linear, or nearly so, with respect to the adsorption, storage and desorption of a broad spectrum of organic components over a wide dynamic concentration range. Although all of the foregoing limitations of present adsorbents have been noted, despite the long-felt need for improved sorbent tubes none have been forthcoming.
Although our invention is simple, it is an extraordinarily effective solution to the foregoing problems and fills a commercial void which has existed for more than a decade. What we have found is that when the sorbent tubes of the purge and trap unit use certain molecular sieves as adsorbents one can very effectively and efficiently adsorb volatile and semi-volatile organic compounds found in contaminated water. Furthermore, the capacity of these molecular sieves as adsorbents in sorbent tubes mandated for use by the EPA is sufficient that "breakthrough" through saturation by components at high concentrations is rarely a threat. Complete desorption of all components is readily attained with avoidance of hysteresis effects because one can utilize substantially higher desorption and bakeout temperatures than are available to the prior art sorbent tube materials; one need not worry about analytical errors arising from materials remaining in a sorbent tube from prior analyses. The resulting benefit is better run-to-run reproducibility and a higher precision in measurement evidenced by a lower relative standard error of deviation. Another benefit is longer sorbent tube lifetime. That is, our adsorbents can undergo more adsorb-desorb cycles than those in the sorbent tubes presently in commercial use. But perhaps most important of all is that the sorbent tubes of this invention exhibit linearity in absorption, linearity in storage, and linearity in desorption, and exhibit such linearity over a dynamic range which easily spans four orders of magnitude. Thus calibration becomes an infrequent occurrence. These cumulative benefits are substantial, and offer advantages over the prior art sorbent tubes which are advantages in kind rather than advantages in degree.